Continuous extrusion of thermoplastic materials

ABSTRACT

This invention relates to a method of continuously extruding thermoplastic material with a solid, hollow or open cross-section, in which a liquid medium is continuously applied to the surface of the material being extruded before it leaves a nozzle which shapes the cross-section of the extruded material to the desired profile, and the surface of the material being extruded is also roughened, either before, simultaneously with or immediately after the application of the liquid medium, but before the material leaves the nozzle.

This invention relates to a method of continuously extruding athermoplastic material with a solid, hollow or open cross-section,particularly cross-sections with a sharp edged profile, and to anextrusion die for use in the method.

The extrusion of non-foamable thermoplastic materials to give productswith a wide variety of cross sectional profiles has been carried out fora long time. It has also been known for a long time to facilitate themovement of the thermo-plastic material through the profiling die of theextrusion machine by introducing a lubricant between the material beingextruded and the highly polished walls of the die. The lubricant spreadsover the surface or surfaces of the strand being extruded, forming afilm.

It is also known to form by extrusion continuous strands of foamedthermoplastic materials having solid, hollow or open cross-sectionalprofiles. A process is also known for manufacturing continuous profiledstrands which have a foamed core enclosed in a jacket which has a smoothsurface. In this process two substances are extruded simultaneously, onebeing foamable thermoplastic material to form the core, and the otherbeing a non-foamable thermoplastic material to form the jacket (seeGerman Patent Specification 1,154,264). In this way there is obtained astrand of foamed plastics material enclosed in a hard, smooth outerskin. However, the equipment required for carrying out this process iscomparatively complex and expensive, and adhesion between the core andthe jacket is not always satisfactory.

A process is also known for the manufacture of a continuous strand offoamed thermoplastic material having a uniform cell structure and asmooth outer surface in which the foamable plastics material is extrudedthrough a die containing an internal mandrel pin directly into a cooledcalibrating device which has the same cross section as the outlet fromthe die. The product therefore has the same cross sectional outer shapeand size as the calibrating device (see German Auslegeschrift1,729,076). What happens in this process is that, as soon as thefoamable material leaves the die outlet, it comes into contact with thecooled calibrating device, the temperature of which is below thestiffening temperature of the foamable plastics material, and a hardlayer forms on the surface of the extruded strand. After this, thematerial is compelled to foam inwardly. With this process, solid, orhollow cross-sectional profiled continuous strands can be extruded usingfoamable thermoplastics materials, the nature of the product dependingon the extrusion conditions, which includes the pull-away speed (i.e.the speed at which the product is pulled away from the extrusion die). Adisadvantage of this process is that, when products are being made whichhave a relatively low bulk density, it is difficult to propel or pullthe foaming extruded strand through the calibrating device. A further,and even more serious, disadvantage is that only low pull-away speedscan be used, usually of the order of 20 to 30 cm/minute, and at most 75cm/minute.

According to the present invention, in a method of continuouslyextruding thermoplastic material with a solid, hollow or opencross-section, a liquid medium is continuously applied to the surface ofthe material being extruded before it leaves a nozzle which shapes thecross-section of the extruded material to the desired profile, and thesurface of the material being extruded is also roughened, either before,simultaneously with or immediately after the application of the liquidmedium, but before the material leaves the nozzle. The roughening of thesurface of the material being extruded has the effect that the liquidmedium penetrates at least partly through the surface of the material,changing the properties of this surface in a desired manner.

The method in accordance with the present invention makes it possible,by simple means, to influence the surface characteristics of continuousextruded strands of foamed or non-foamed thermoplastics material, notonly by forming a smooth outer skin but also in other ways such asgiving the surface a desired colour. More important, however, thepresent invention makes it possible to extrude foamable thermoplasticsmaterial using pull-away speeds of at least 5 m/minute, the productsimultaneously being given a smooth outer skin. The method in accordancewith the invention is particularly suitable for extruding products whichhave complex profiles and which have bulk densities lower than 0.15,preferably lower than 0.1. Also, if an extrusion nozzle is used havingan internal mandrel pin to produce an extruded strand having a hollowcross-section, the method can be arranged so that the extruded strandcan be subdivided into several separate products simply by tearing thestrand apart longitudinally.

In accordance with a further aspect of the invention, an extrusion diefor use in carrying out the method comprises a nozzle through which thematerial is extruded to provide it with a desired cross-sectionalprofile, a tubular nozzle holder having a passage through which thematerial passes to the nozzle and which has a roughened surface upstreamof the nozzle, and feed ducts through which a liquid medium can beapplied to the surface of the material passing through the die upstreamof the nozzle.

For producing hollow cross-sectioned extruded strands, the extrusionnozzle is provided with an internal mandrel pin supported in position bypin holding means. The tubular nozzle holder, of which its internalpassage is arranged coaxially with the nozzle, may be provided withheating devices, and temperature and pressure sensors for assisting thecarrying out of the method in accordance with the invention.

The feed ducts through which the liquid medium is applied to the surfaceof the material being extruded may be located in the nozzle holderand/or the nozzle. Also, feed ducts may be located in the mandrel pinand/or the pin holding means when the nozzle is provided with these.

The liquid medium used in the method should be a substance which isfluid or pasty at the temperatures and pressures prevailing in thenozzle holder during the method, and which preferably has a lowviscosity under these conditions. In general the liquid media preferredare fluid, highly viscous, waxy or solid at room temperature (excludingthermoplastic synthetic substances), and are fluid or pasty at thetemperatures and pressures prevailing in the nozzle holder and nozzleduring the method. Usually, the boiling point of the liquid medium ishigher than the temperature of the plastic mass being extruded, that isto say higher than the temperatures in the nozzle holder and nozzle,although under certain special conditions it is possible to use a liquidwhich has a lower boiling point than this. As an example, in theextrusion of profiled sections from foamable thermoplastic materials, amatt-finished surface skin may be produced by using as the liquid mediuma liquid which has a comparatively low viscosity at room temperature andhas a boiling point of at least 140° C, preferably over 200° C.Substances which are soluble in water are particularly useful as liquidmedia, for example, water-soluble silicon oils of the kind used inpolyurethane chemistry as foam stabilizers.

The liquid medium used in accordance with the invention serves either tosolve a special problem or simultaneously to solve several problems, inwhich case additives may be carried by the liquid medium which arealready known in existing extrusion techniques. Therefore, they need notbe described here, but as one example, the liquid medium may carry a dyefor colouring the surface of the product, in which case the mediumshould be capable of acting as a solvent for the particular dye.

The quantity of liquid medium to use depends on its intended function.In practice, it has been found that the quantity to use must bedetermined experimentally in each particular case. To produce a smoothskin on a foamed extruded product there may be used, depending on thecircumstances, between 3 and 12 g/m² of a liquid medium, preferablybetween 6 and 8 g/m², based on the external surface area of the product.In many cases a considerable portion of the liquid medium will beallowed to remain on the surface of the product, as such an excess isoften desired for practical reasons. On the other hand, an undesiredexcess of the liquid medium can easily be removed from the surface ofthe product, at least in the case of a water-soluble substance, byapplying a spray of water.

The liquid medium is introduced under a pressure which is directlyproportional to the quantity it is intended to use. In general, thepressure of injection is 10 to 30% higher than the pressure at the innersurface of the nozzle holder or nozzle due to the material beingextruded.

The temperature of the liquid medium when it is being injected isusually nearly the same as the temperature of the plastic mass in theextruder, the temperature of the liquid medium being raised by the warmwalls of the feed ducts through which it flows. If a liquid medium isused which is highly viscous, waxy or even solid at room temperature, itmust of course be heated enough before being injected through the feedducts to ensure that it is sufficiently fluid. In certain cases,however, the liquid medium may be used for cooling the surface of theproduct. In this case the liquid medium may be injected at roomtemperature or below and it must have a low viscosity. The cooled liquidmedium can be injected through a thermally insulated intermediate partwhich contains the necessary bores or an annular nozzle.

The liquid medium is preferably introduced into contact with the surfaceof the thermoplastic material in the nozzle holder at several oppositelocations, preferably through very small diameter bores and/or anannular groove extending around the surface of the passage in the nozzleholder and into which the feed ducts open.

Liquid media suitable for use in the method in accordance with thepresent invention include mineral oils and fats, plant and animal oilsand fats, sulfonated mineral oils and fats, sulfonated plant and animaloils and fats, silicone oils, particularly water-soluble silicone oils,liquid soaps, triethanolamine, low-molecular polyesters and polyethers,waxes, glycerin, liquid prepolymers of the thermoplastic material beingextruded, and organic and inorganic solvents in which the thermoplasticmaterial is soluble or insoluble, depending on the nature of the problemtackled. Also, under certain circumstances, polymerisable reactionmixtures which take a long time to set can be used as the liquid medium.These substances polymerize after the product has left the nozzle,producing a smooth outer surface.

Examples of the method and of the extrusion die in accordance with theinvention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic longitudinal section through one form ofextrusion die mounted on the end of an extruder;

FIG. 2 is a diagrammatic longitudinal section through an alternativeform of mandrel pin and its holders for use in the nozzle of the dieshown in FIG. 1;

FIG. 3 is a diagrammatic longitudinal section through a portion of thenozzle holder of the die of FIG. 1, showing the arrangement of the feedducts for the liquid medium;

FIG. 4 is a view similar to FIG. 3, but showing an alternativearrangement for the feed ducts; and,

FIG. 5 is a diagrammatic cross section through a product having acomparatively complex hollow profile extruded by a method in accordancewith the invention.

The extrusion die shown in FIG. 1 is mounted on the downstream end of aconventional extruder, of which the drawing shows only the end-piece 12and the jacket 13 containing the extruder worm 14. The internal passagein the end-piece 12 at first decreases in diameter conically, in thedownstream direction, and is then cylindrical as far as the outlet 15,which is followed by a screen 11 backed by a perforated plate 10. Thedie is attached to the downstream end of the end-piece 12, and comprisesa nozzle holder 4 supporting, at its downstream end, a profilednozzle 1. Penetrating radially inwards through the wall of the nozzleholder 4 there are feed ducts 5 through which, in use, a liquid mediumis fed in from injection channels 6. The nozzle holder 4 and, ifdesired, also the nozzle 1, is equipped with a heater 7, a temperaturesensor 8 and a pressure sensor 9. In this example the profiled nozzle 1contains an internal mandrel pin 2 supported by four mandrel pin holders3 in the form of thin bridges spaced at 90° intervals around thecircumference.

When the apparatus is in operation, a foamable plastic mixture, whichmay for example consist of polystyrene, a foaming agent and conventionaladditives, is propelled by the worm 14 through the end-piece 12 and outthrough the screen 11 and the perforated plate 10 into the internalpassage of the tubular nozzle holder 4. During its passage through thenozzle holder 4 the foamable plastic mixture is given a surfacetreatment by injecting a liquid medium through the feed ducts 5.

The length of the nozzle holder 4 depends on the pull-away speed, thatis to say the speed at which the extruded product is pulled away fromthe nozzle 1, on the desired distribution of the liquid medium over thesurface of the product and on other extrusion conditions. From thenozzle holder 4 the foamable plastic mixture is forced through theprofiled nozzle 1, and it should be noted that the extrusion conditionsare selected so that the mixture does not foam during its passagethrough the nozzle holder 4, does not foam or foams only slightly on itsway through the profiled nozzle 1, and only begins to foam properly whenit leaves the outlet of the nozzle 1. Consequently, as soon as it leavesthe profiled nozzle 1, the plastic mixture, the cross section of whichat this instant is determined by the profile of the nozzle, begins tofoam in the open air and a smooth outer skin 17 is formed, asrepresented for example in FIG. 5. The plastic mixture foams bothoutwards and inwards, the increase in the volume of the foam dependingon a number of factors which must, in practice, be determinedexperimentally, particularly in the case of complex profiles. In themanufacture of foamed plastic profiled extrusions which are foamedfreely in the air, it is a known guiding rule that the dimensions of thenozzle 1 should be about 1/3 of the dimensions of the desired profiledproduct, assuming optimum extrusion conditions (composition of thefoamable plastic mixture, temperatures, pressures, pull-away speed andthe like). If desired, a product made by a method in accordance with theinvention can be given sharper contours by controlling the foaming ofthe plastic material using a calibrating device of progressivelyincreasing internal cross section, preferably made of a highly polishedchrome steel, downstream of the nozzle 1. The foaming angle, or theeffective length of the calibrating device, is determined essentially bythe degree of difficulty of manufacture, by the desired sharpness of theresulting edges and by the volume of the profile, particularly in thecase of ornamental beadings or covings. Further advantages may beobtained by passing the foamed product between rollers or the like in aknown manner. This is usually done while the extruded product is stillwarm and deformable. The product is pulled away by means of tractionchains, as is also known, and finally the extruded profiled strand iscut into lengths with a saw.

In the manufacture of profiled extrusions having a hollow cross section,it is sometimes desired to give the internal surface of the product asmooth skin. In this case, the internal mandrel pin 2, which may alsohave a roughened surface, preferably has internal feed ducts (not shown)which communicate through the bridge-like mandrel pin holders 3 and thenozzle holder 4 with the injection channels 6. In another version of themandrel 2 as shown in FIG. 2, feed ducts 3a are provided only in themandrel pin holders 3.

Although the feed ducts 5 will usually only be in the nozzle holder 4,similar feed ducts may be provided through the walls of the nozzle 1,either as well as or instead of the ducts 5 in the walls of the nozzleholder 4. The feed ducts 5 are preferably in the form of several boresof very small diameter, wherever they are located be it through thewalls of the nozzle holder 4, the walls of the nozzle 1, the mandrel pinholders 3, or the mandrel 2 itself. If desired, the feed ducts 5 mayopen into an annular groove in the inner surface 4a of the nozzle holder4, the inner surface 1a of the nozzle 1, and/or the outer surface 2a ofthe mandrel pin 2, in which case there need only be one feed duct 5.Preferably however, there are feed ducts 5 only in the walls of thenozzle holder 4, the feed ducts penetrating radially through the wall inthe same cross sectional plane as each other, as represented in FIGS. 1and 3, although in some cases it may be preferred to make the feed ducts5 penetrate at an angle through the walls of the nozzle holder 4, asrepresented in FIG. 4.

The tubular nozzle holder 4 is preferably replaceable and its internalpassage may have any desired cross sectional shape, although this ispreferably circular. The internal passage will therefore usually becylindrical, but it may expand somewhat conically in the direction ofthe nozzle 1.

Particularly good results are obtained if the roughening of the innerpassage of the nozzle holder 4 is provided by very fine annular ridgesor helical rifling, preferably extending over the entire inner surface4a. In certain cases however, it is advantageous to roughen the innersurface of the nozzle holder 4 only in the region between the nozzle 1and the feed ducts 5, these ducts 5 preferably being in the upstreamhalf of the nozzle holder 4.

The length of the nozzle holder 4 depends on the particular method inoperation, but in general the nozzle holder 4 should have a length atleast 3 times, preferably at least 8 times, its internal diameter.

The method in accordance with the invention is applicable to all theknown processes for extruding solid, hollow or open cross-sections fromeither foamable or non-foamable thermoplastic materials. When usingfoamable plastics, either chemical or physical (direct gas or vapourinjection) foaming agents can be used, or a combination of the two.Direct gas injection is of economic interest only in the manufacture ofproducts with bulk densities of 100 km/m³ or less. Such low bulkdensities have not hitherto been practicable using chemical foamingagents. Direct gas injection is used in a particularly usefulapplication of the present invention, i.e. in the manufacture of verylight ornamental profiles made of foamed polystyrene.

Thermoplastic materials which may be used in the method are polymers orcopolymers of vinyl or vinylidene monomers such as ethylene, propylene,butadiene, styrene, vinyltoluene, alphamethylstyrene, acrylonitrile,vinyl chloride, vinyl acetate, methyl acrylate, methyl methacrylate, orethyl acrylate. Particularly preferred are polyvinylaromatic plastics,that is to say polymers or copolymers of vinylaromatic monomers such asstyrene, chlorostyrene, vinyltoluene or alpha-methylstyrene. Thecopolymers may be made from vinylaromatic monomers together with adifferent olefinic monomer such as acrylonitrile, vinyl chloride, vinylacetate, methyl acrylate, methylmethacrylate or ethyl acrylate.Toughened polystyrenes may also be used in the method in accordance withthe invention. Excellent results are obtained using polystyrene. Amongother synthetic materials which may be used in the method are polyamidesand the like.

When foamable thermoplastic material is used, the foaming agent added tothe basic thermoplastic material may be a chemical substance whichdecomposes at certain temperatures with the formation of gases, or itmay be a volatile substance which is a gas or vapour at normaltemperatures and pressures (20° C, 1 atm), but which dissolves in themolten or semi-molten thermoplastic material at the temperatures andpressures existing in the extruder. Among the volatile substances whichmay be used are low-molecular aliphatic hydrocarbons such as ethane,ethylene, propane, propylene, isobutene, butadiene, butane, isopropene,or pentane, low-molecular alkylhalogenides such as methyl chloride,dichlorodifluoromethane, trichloromethane,1,2-dichlorotetrafluoroethane, and inorganic gases such as carbondioxide or nitrogen. Preferred are the low-molecular aliphatichydrocarbons, particularly butane, isobutene and the frigenes, that isto say monofluorotrichloromethane, difluorodichloromethane,trifluoromonochloromethane, monofluorodichloromethane,difluoromonochloromethane, 1,2,2-trifluorotrichloroethane,1,1,2,2-tetrafluorodichloroethane, octafluorocyclobutane,trifluorobromomethane and mixtures of these substances.

The foaming agent is usually used in concentrations of 3 to 50%,preferably 7 to 30%, based on the weight of the thermoplastic material,the concentration used depending of course on the bulk density desiredfor the foamed product. If direct injection of gas or vapour is used,the substance is continuously sprayed into the molten plastic in theextruder. Another method for producing foam is to add a previouslyprepared composition which already contains a foaming agent.

If it is desired to produce a particularly fine cell structure in thefoamed extruded product, it is advisable to add to the material to beextruded substances which form nuclei. A number of compounds aresuitable for this purpose, for example finely divided inert solids suchas silicon dioxide or aluminium dioxide, if necessary together with zincstearate or small quantities of a substance which decomposes at theextrusion temperature with the formation of gas, for example sodiumcarbonate, if desired together with acetic or citric acid. The substanceis added in a concentration up to 5%, based on the weight of thethermoplastic material, so that an intimate mixture is formed. Thenuclei-forming substances which may be used are well known in the artand need not be described in any further detail.

On the other hand, the foaming agent may, as already mentioned, be achemical foaming agent, for example a bicarbonate such as sodiumbicarbonate or ammonium carbonate. Other examples are ammonium nitrite,or an organic nitrogen compound which releases nitrogen on heating, forexample dinitrosopentamethylendiamine, barium-azodicarboxylate,azodicarbonamide, substituted thiatriazole,diphenylsulfone-3,3'-disulfonehydrazide or azoisobuteric acid dinitride.

The extrusion temperature (that is to say the temperature of theextruder and the thermoplastic material in the extruder) depends to acertain extent on the softening point of the plastic material to beextruded. Usually temperatures of 95° to 190° C are used, more often100° to 160° C. For example, in extruding foamable polystyrene thetemperature is between 130° and 160° C. Polyethylene requires a ratherlower temperature, between 95° and 110° C.

The pressure in the extruder is usually sufficient to prevent theplastic mixture from beginning to foam before leaving the extrusion die.If the foaming agent is a condensible, volatile substance the pressureshould be greater than the saturated vapour pressure of the volatilesubstance at the extrusion temperature. Quite high pressures, forexample between 17 and 105 kg/cm², may be used. The pressure ispreferably between 21 and 70 kg/cm².

The following are only a few examples of the many in accordance with theinvention which may be used to produce foamed extruded products having aprofiled smooth outer skin.

EXAMPLE 1

This example describes the manufacture of a solid cross-section foamedpolystyrene rod with a smooth silky-mat surface. The rod has ahomogeneous surface skin approximately 0.3 mm thick, and a bulk densityof 0.11 (110 kg/m³).

The polystyrol for extruding and foaming is mixed for about 3 minutes ina rapidly rotating mixer with, by weight, 0.95% of citric acid, and 1.2%of sodium bicarbonate. The mixture is then introduced in the usual wayinto a double worm extruder and heated to 190° C, to make the mixtureplastic. Into the plastic mass there is then injected continuously atthe point where the pressure has almost reached its highest value 7% ofisopentane, with sustained mixing and kneading. The resulting foamableplastic mixture is then driven through the nozzle holder 4 and thenozzle 1 of an extrusion die similar to that of FIG. 1 but without anymandrel pin, so that extrusion takes place. The mixture is cooledstepwise down to 120° C in the extrusion die, and during the extrusion awater-soluble silicon oil is injected through the feed ducts 5 in thenozzle holder 4. The extruded polystyrene strand foams homogeneously inthe air. The product is conveyed by a pull-away device, and with thehelp of supplementary rollers, through a cooling bath. The pull-awayspeed is 5.5 m/min. and the cooled strip is subsequently cut to desiredlengths by a sawing device.

Rods of other bulk densities may be made in the same way withoutchanging the original mixture, for example by changing the pull-awayspeed, the size of the extrusion nozzle, and the spacing between therollers.

EXAMPLE 2

This example describes the continuous manufacture of a very low bulkdensity strand having a hollow profiled cross-section, such as of thekind represented at 20 in FIG. 5. The foamed material is polystyrene andthe bulk density of the product is 0.06 (60 kg/m³). The product has asmooth outer skin 17, a foam layer 19 and separation planes of weakness18.

The procedure is as for Example 1, but modified in that instead ofisopentane there is used frigen, and instead of the pinless mandrel oneuses an internal mandrel pin supported by four mandrel pin holdersangularly spaced at 90° as in FIG. 1. Furthermore, when the extrudedproduct emerges from the nozzle 1 into the air its temperature is onlyabout 100° C. A silicon oil is injected continuously through the feedducts 5 in the nozzle holder 4, at a rate of 6 g/m² of outer surface ofthe hollow profile, and as soon as the product emerges from the nozzleit already has a semi-matt surface skin. The injected silicon oilfulfils two requirements. It not only forms the smooth outer skin on theproduct but also, being injected at a sufficiently rapid rate,distributes itself over the surfaces of the mandrel pin holder by whichthe flowing plastic mass is subdivided into four strips. The siliconeoil on the surfaces of the holder prevents the four strips fromsubsequently adhering firmly together on passing through the nozzle, andhence the separation planes 18 are formed. The finished product is easyto tear apart to form four separate strips, without damage and withoutthis requiring the use of a special tool. These strips are used asornamental ceiling covings.

EXAMPLE 3

Example 1 is repeated, with the modification that the polystyrene ismixed with 1.5% of magnesium silicate and 2.5% by weight of kieselgur.Both these substances have previously been ground very fine and roasted.Foaming is produced by 17% of ethyl chloride. The plastic mass isextruded into the air at a temperature of 134° C. The product, which issawn into lengths as desired, is a profiled plastic moulding with asmooth, hard and continuous outer skin, and a bulk density of 0.39 (390kg/m³).

EXAMPLE 4

Example 1 is repeated, but with the modification that polyethylene isused instead of polystyrene. The material is extruded into the air at80° C. The product is profiled with a smooth outer skin and a bulkdensity of 0.21 (210 kg/m³).

What we claim is:
 1. An extrusion die attachable to means forcontinuously extruding thermoplastic material having a preselectedcross-section, the die comprising a nozzle through which the material isextruded to provide it with a desired cross-sectional profile, anelongate tubular nozzle holder having a wall defining an internalpassage through which the material passes to the nozzle, and feed ductsupstream of said nozzle penetrating through said wall and opening intosaid passage through which a liquid medium can be applied to the surfaceof the material passing through the die, said wall having a roughenedinternal surface between said nozzle and said feed duct penetration. 2.An extrusion die according to claim 1 wherein said penetration of saidwall by said feed ducts is located in the upstream half of said nozzleholder and substantially the entire length of said nozzle holderinternal wall between said nozzle and said feed duct penetration has aroughened surface.
 3. An extrusion die according to claim 1, in whichthe feed ducts are in the form of very small diameter bores.
 4. Anextrusion die according to claim 1, in which the nozzle has an internalmandrel pin supported in position by pin holding means.
 5. An extrusiondie according to claim 1 in which the nozzle holder passage has acircular cross section.
 6. An extrusion die according to claim 5, inwhich the nozzle holder passage is cylindrical.
 7. An extrusion dieaccording to claim 5, in which the nozzle holder passage is conical,expanding towards the nozzle.
 8. An extrusion die according to claim 5,in which the nozzle holder passage is at least three times as long asits diameter.
 9. An extrusion die according to claim 1 in which theroughened surface of the nozzle holder passage is provided by very fineannular ridges.
 10. An extrusion die according to claim 1 in which theroughened surface of the nozzle holder passage is provided by very finehelical rifling.
 11. An extrusion die according to claim 4 wherein saidmandrel pin has a roughened outer surface portion and feed ductspenetrating through said outer surface of said mandrel.
 12. An extrusiondie according to claim 1 in which the feed ducts open into an annulargroove extending around the surface of the passage.
 13. An extrusion dieaccording to claim 1 in which the feed ducts penetrate radially inwardsthrough the walls of the nozzle holder, and are all in the samecross-sectional plane.